From combating COVID-19 and climate change to tackling corruption and tax evasion, international organisations (IOs) play a critical role in helping countries find solutions to common problems. But for IOs to deliver optimal support to countries and their populations, the international instruments they develop need to be inclusive, well understood and have a tangible impact. This Compendium of International Organisation’s Practices gathers the experiences of some 50 IOs with different mandates, members and institutional frameworks to draw lessons for international rulemaking. It examines the diversity of instruments in the international rulemaking ecosystem; describes trends and challenges; and discusses how to strengthen the implementation and evaluation of international instruments, ensure efficient stakeholder engagement, and maximise opportunities for co-ordination across IOs. The IO Compendium is designed to serve not only all national and international policy practitioners, but also civil society actors, academic experts, private actors, and citizens who seek to understand, benefit from and possibly contribute to the international rulemaking process.